On achievements that require you to do certain things with certain races, criteria matching does not always work in languages with grammatical gender. For example, for the "Check Your Head" achievement for Hallow's End, you have to throw pumpkins at players of each of the listed races. However, in the criteria in German, blood elves are listed as "Blutelf(e)" and night elves are "Nachtelf(e)", so when mousing over a blood elf or night elf player, the achievement need/done line isn't added to the tooltip.

I'd guess this would also affect achievements like "Let It Snow" (link to German version) that list race/class combos like "Menschlicher Krieger" that won't match what's in a unit tooltip ("Mensch - Krieger" in the default tooltip) and won't match what's returned by UnitRace (just "Mensch") or what's returned by UnitClass for female characters ("Kriegerin").

It probably also affects other gendered languages. I'm not sure how much effort you want to put into this, but there's probably some pattern matching magic you could do...